1. Field of the invention
The present invention relates to toner particles employable for producing a visible image from a latent image in an electrostatographic or magnetographic recording method.
2. Description of prior art
As the process for fixing a toner image in a recording method such as electrostatography, there have been known three fixing processes, that is, a heat fixing process, a solvent fixing process and a pressure fixing process. Recently, the heat fixing process and the pressure fixing process, both using no solvent, are widely used from the viewpoint of the prevention of environmental pollution.
In the heat fixing process, a toner comprising a colorant with a binder has been conventionally employed. The conventional toner comprises colorant such as carbon black dispersed in a binder. Such conventional toner has been also employed in the pressure fixing process. However, utilization of an encapsulated toner that is in the form of the micro-capsule containing a colorant in its core portion is recently proposed in the pressure fixing process.
The encapsulated toner is a toner in the form of micro-capsule prepared by enclosing a core material comprising a colorant such as carbon black and a binder such as a polymer and/or an oily liquid with a resin shell which is rupturable by application of pressure.
The known encapsulated toner and the conventional non-encapsulated toner are not necessarily satisfactory in various properties essentially required for electrostatography or magnetography.
For example, a toner employable as a developing agent in electrostatography is required to have various excellent properties such as high powder flowability, high developing efficiency, and no smearing or staining of the surface of a photosensitive member for producing a latent image. This is called "offset phenomenon". Further, in the case of a two-component developing process, it is necessary that the toner does not smear the surface of the employed carrier. In the pressure fixing process, high fixability, little occurrence of offset phenomenon on a press roller used in the process (that is, toner adheres to the surface of press roller to stain the roller), etc. are also required for the toner.
Accordingly, the toner employed in the pressure fixing process should be satisfactory in all properties such as powder flowability, fixability to a supporting medium (e.g., paper), preservation stability of the fixed image, anti-offset property, and electrostatic chargeability and/or conductivity required depending upon a developing process. However, the conventional toners are not well satisfactory in the above-mentioned characteristics.
For instance, decrease of fixing energy is recently desired, because there is a demand to minimize volume of a total system of a duplicating apparatus. In the heat fixing process, it is desired to lower the softening point of fixing components of toner such as polymer and wax for decreasing the fixing energy. However, lowering of the softening point of the component sometimes causes agglomeration of toner particles, because the temperature inside a duplicating apparatus (i.e., copying machine) sometimes increases to a level of from 50.degree. C. to 60.degree. C. due to accumulation of heat of each parts of the apparatus. Moreover, the toner in which a binder component having a low softening point is utilized is apt to adhere to a surface of a heat roller, that is, to cause the offset phenomenon.
The decrease of fixing energy in the pressure fixing process can be accomplished by decreasing the fixing pressure, together with using a toner which is fixable at a low pressure. Such toner can be prepared using a polymer or wax having a low softening point. Such lowering of softening point of a component of a toner is apt to cause the toner agglomeration and offset phenomenon.
The previously proposed encapsulated toners still have the above-mentioned disadvantages, depending on the material of polymer forming the shell.
To solve these problems and improve the flowing property of toner, the encapsulated toner whose shell has a colloidal silica is proposed in Japanese Patent Provisional Publication No. 54(1979)-76233. The toner having a colloidal silica thereon shows an improved flowing property, so long as the toner is not kept for a long period of time. However, this toner is apt to form an agglomerated mass when it is stored for a long period of time at a time temperature such as 60.degree. C. Thereofore, a toner having colloidal silica still has a problem in its storage stability. Also known is a toner having on its surface silica (prepared by a dry process through oxidation of a silicon halide in a vapor phase) which has been treated with a titanate-type coupling agent (i.e., titanate coupling agent), as is described in Japanese Patent Provisional Publications No. 59(1984)-123850. The silica having been treated with a titanate coupling agent serves to improve positive electrochargeability, but is poorly effective to prevent the agglomeration of toner at a high temperature and the offset phenomenon of toner.